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This paper describes an efficient charging method for a supercapacitor-operated, solar-powered wireless sensor node called Everlast. Unlike traditional wireless sensors that store energy in batteries, Everlast's use of supercapacitors enables the system to operate for an estimated lifetime of 20 years without any maintenance. The novelty of this system lies in the feed-forward, pulse frequency modulated converter and open-circuit solar voltage method for maximum power point tracking (MPPT), enabling the solar cell to efficiently charge the supercapacitor and power the node. Experimental results show that by its low-complexity MPPT, Everlast can achieve over 89% conversion efficiency with lower power overhead than the state-of-the-art by two orders of magnitude, while enabling charging a supercapacitor up to 400% faster than direct charging. This makes Everlast particularly applicable to miniature-scale, high-impedance energy harvesting systems.
Simjee et al. (2008) studied this question.